(1) Field of the Invention
The present invention relates to pitching stabilization means and to a rotary wing aircraft provided with such pitching stabilization means, in particular to minimize the attitude hump phenomenon. The invention is thus situated in the narrow technical field of aircraft that are subjected to the attitude hump phenomenon.
(2) Description of Related Art
Conventionally and by way of example, a helicopter comprises an airframe extending longitudinally from a front end towards a tail end on either side of an anteroposterior plane of symmetry, and in a vertical direction from a bottom portion fitted with landing gear to a top portion fitted with a main rotor for providing lift and propulsion.
Furthermore, the airframe includes a tail rotor at its tail end. A particular function of the tail rotor is to oppose the yaw torque exerted by the main rotor on the airframe. The tail rotor also enables yaw movements of the helicopter to be controlled.
A helicopter sometimes also includes additional stabilizer surfaces. For example, it is common practice to fit a helicopter with a yaw stabilizer surface, e.g. contained in the anteroposterior plane, in order to oppose said yaw torque at high forward speeds of the aircraft.
Such a yaw stabilizer surface is sometimes referred to as a “fin”.
Likewise, a helicopter sometimes includes pitching stabilization means presenting an angle of absolute value lying in the range 0° and plus or minus 90° relative to said anteroposterior plane, the pitching stabilization means comprising, by way of example, two pitching stabilizer surfaces extending symmetrically on either side of said anteroposterior plane.
Such pitching stabilization means are sometimes referred to as a “horizontal stabilizer” or more simply as a “stabilizer”. The term “stabilizer” is frequently used on its own given that the stabilization means are not necessarily horizontal.
The pitching stabilization means are very effective, with their effectiveness increasing with increasing forward speed of the helicopter. Furthermore, it can be understood that the effectiveness of pitching stabilization means is maximized by maximizing their wing area.
However, the flow of air passing through the main rotor of a traditional helicopter in flight is deflected towards the tail and impacts against the pitching stabilization means, in particular when moving at a slow speed in translation or when hovering. This flow of air then exerts forces on the pitching stabilization means that the pilot compensates by operating the flight controls.
Nevertheless, when conditions of flight vary, the deflection of the airstream is also modified. The same therefore applies to the forces exerted on the pitching stabilization means.
This phenomenon is referred to as the “attitude hump”.
During a stage of acceleration or of deceleration, the forces exerted by the flow of air passing through the main rotor and impacting against the pitching stabilization means tend to cause the helicopter's nose to rise. In order to stabilize the speed of the helicopter, the pilot then needs to use the cyclic pitch control stick to control the blades of the main rotor in order to diminish the nose-up attitude of the helicopter.
It can be understood that optimize the pitching stabilization means by maximizing their wing area makes the attitude hump worse.
Under such circumstances, it appears to be impossible to use pitching stabilization means having large wing area on a helicopter without giving rise to an increase in the attitude hump phenomenon.
In order to remedy that, stabilization means are known having a stationary stabilization surface and a movable surface. The position of the movable surface relative to the stationary stabilization surface can then be controlled using flight controls in order to limit the attitude hump phenomenon.
Although advantageous, that configuration needs flight controls, thereby giving rise to an increase in the weight of the aircraft, and also to increased complexity of the aircraft.
A second solution consists in arranging pitching stabilization means for a rotary wing aircraft outside the wake of the rotary wing. Nevertheless, that second solution can in fact lead to a significant loss in the effectiveness of the stabilization means.
For this purpose, document FR 1 511 006 describes an aircraft having a rotary wing and a horizontal stabilizer that is arranged at the top of a fin.
An object of the present invention is thus to propose pitching stabilization means for a rotary wing aircraft seeking to minimize the attitude hump phenomenon.
In the technological background remote from the technical field of the invention and the technical problem of minimizing the attitude hump phenomenon, high-lift flaps are known.
A wing conventionally has a stationary airfoil surface. Certain airplanes then include firstly a stationary airfoil surface and secondly high-lift flaps that extend the airfoil surface in the forward direction of the airplane in order to minimize the stalling speed of the airplane.
Each high-lift flap is movable to allow air to pass between the stationary airfoil surface and the flap.
Document FR 2 864 026 relates to a method and a device for reducing the vibration that is generated on the structure of a helicopter by the air flow coming both from the main rotor that provides the aircraft with lift and propulsion and from the flow of air along the fuselage.
Document U.S. Pat. No. 3,902,688 describes an aircraft having a horizontal stabilizer arranged at the top of a fin, and a horizontal stabilizer arranged at the bottom of the fin.
Document U.S. Pat. No. 3,583,659 describes a helicopter having a tail rotor.
The helicopter has a fairing constituted by a plurality of mutually parallel stationary rings.
Document U.S. Pat. No. 4,828,203 describes an aircraft provided with:
a fuselage;
a set of canard foils fastened to the fuselage in front of the center of gravity of the aircraft;
a set of lift-providing ducted rotors attached to the fuselage level with said center of gravity; and
a fixed wing fastened to the fuselage behind the center of gravity of the aircraft.
Under such circumstances, that document is remote from the invention because it relates to a fixed wing aircraft and not a rotary wing aircraft. In addition, the ducted rotors are not stabilizer type pitching stabilization means, but members provided with propulsion means.
The term “pitching stabilization means” is used herein to mean passive means not having a fan or an engine.
Document U.S. Pat. No. 4,227,665 describes a stabilization surface including a slot.
The following documents are also known: EP 2 409 917, WO 2011/034531, U.S. Pat. No. 6,352,220, and U.S. Pat. No. 2,464,663.